Great game
First, the good:
It was good, simple, bloody fun. The sound was fantastic, and I am pretty sure the artwork was original (and well done, I love the old fashioned pixelized guys, it really makes for good animation).
Now the bad.
The only issues I have with it are as follows:
When you begin a level with someone spawning underneath you (which is 90% of the time) damage is inevitable, and irritating.
The difficulty in acquiring points (thus upgrade) is extreme. It takes you two or three levels to even upgrade your gun.
The "retry" button doesn't start the level over, it starts the whole game over. This is frustrating.
You can't jump over zombies, they damage you anyway. Thus, the jump function is useless.